This invention relates to digital encoding, storage, and retrieval of analog composite color video signals, e.g., NTSC signals, for use in, for example, TV still frame systems, digital TV receivers, and electronic publishing systems.
Digital encoding of such signals enables video images to be stored and retrieved at will. Even when not stored, such digitally encoded images have advantages over their analog counterparts.
Any digital encoding and retrieval system needs to allow recapture of the frame synchronization, color burst, and picture information that appeared in the horizontal scan lines of the original analog NTSC signal. Also, when the images are to be stored, the system must provide a memory addressing scheme for assigning the digitized information to locations in memory.
Digital encoding typically involves independently sampling each of the red, green, and blue components of the picture signal, a scheme which requires a substantial amount of memory space.
Kashigi, U.S. Pat. No. 4,325,075, discloses a different encoding technique in which the original composite NTSC signal is directly sampled to generate 576 samples for each of 512 horizontal scan lines. The samples are stored in random access memory (RAM) using a complicated RAM address mapping scheme.
It is known that the sampling rate for digital encoding should be at least as high as the Nyquist sampling rate. In the system described by Kashigi, sampling was done at three times the subcarrier frequency.
Typical video playback devices, whether analog or "digital", use a 3.58 MHz crystal controlled clock and analog "sandcastle" circuitry for setting the phase of the clock based on the color burst information contained in the horizontal scan line. Such playback devices also contain analog circuitry for establishing and maintaining a DC blanking level for the horizontal scan line.